Image sensors find applications in a wide variety of fields, including machine vision, robotics, guidance and navigation, automotive applications and consumer products. In many smart image sensors, it is desirable to integrate on-chip circuitry to control the image sensor and to perform signal and image processing on the output image. Charge-coupled devices (CCDs), which have been one of the dominant technologies used for image sensors, however, do not easily lend themselves to large scale signal processing and are not easily integrated with complementary metal oxide semiconductor (CMOS) circuits.
Both passive pixel sensors (PPS) and active pixel sensors (APS) may be made compatible with CMOS technologies. Both PPS and APS are often arranged as arrays of elements, that may be read out, for example, one row at a time. Each row can be read out at one time, driven and buffered for sensing by a readout circuit. PPS include a single transistor (1T) within the pixel unit cell and an amplifier per column located outside of the array. The single transistor is used as a charge gate for switching the contents of the pixel to the column amplifiers. APS includes an amplifier per pixel within the pixel. The in-pixel amplifier of the APS typically converts the pixel output signal to a low impedance and may provide an output signal that is less sensitive to noise on a pixel-by-pixel basis. APS commonly have four transistors (4T), but other configurations (for example, 3T and 5T) are also known. APS have increased readout sensitivity as compared with PPS.
There is interest in manufacturing smaller image sensors with higher spatial resolution, for example by making the pixels smaller. Smaller pixels, however, typically result in reduced photosensitive areas and produce higher image noise and/or poor low light performance. Although APS provide reduced noise pick-up, APS include more transistors within the pixel and thus have a reduced fill factor (a ratio of pixel size over active photosensitive area) as compared with PPS. PPS, on the other hand, may achieve a high fill factor, but typically suffer from a large parasitic capacitance due to the connection lines throughout the pixel array and noise coupling from the substrate through this capacitance.